Many different methods and apparatus exist for coupling integrated circuits to lead frames. Typically the integrated circuit is coupled centrally within the lead frame to form an integrated circuit/lead frame assembly using an adhesive, such as epoxy. The integrated circuit is then electrically coupled to the lead frame assembly. The electrical connection between the integrated circuit and the lead frame are usually thin gold wire soldered at one end to the integrated circuit and at the other end to the lead frame. The central portion of the lead frame assembly is then encapsulated in a plastic or ceramic material so that only the lead pins or terminals of the lead frame extend from the encapsulating material.
It is very important that the adhesive bond between the integrated circuit and the lead frame be sufficiently strong to prevent any shifting of the components during operations performed on the lead frame assembly. It is also important that the gold wires coupled between the integrated circuit and the lead frame be both strong and electrically conductive. Therefore, it is important that the surface of the lead frame to which the integrated circuit and electrical connections attach be very clean prior to the application of adhesive or soldering of gold wire, in order to assure the proper connections. It may also be desirable to clean the lead frame assembly prior to encapsulation of the integrated circuit, because contamination on the lead frame assembly prior to encapsulation may result in inferior or non-functioning products.
Lead frames are typically stamped metal strips and are typically handled by automated machinery in magazines that hold stacks of lead frames, often 25 or 40 in number.
Various prior art apparatus and methods exist for use in cleaning the surfaces of the lead frames, including the use of gas plasmas such as argon plasma. It is known in the art to use removable magazines having perforated sides to hold the lead frames during cleaning. The perforations in the sides of the magazines allow the plasma to enter the magazine to remove unwanted material, such as oxides, from the lead frame surfaces. Typically, the lead frames are loaded into the magazine outside of the cleaning apparatus. Then, at a later stage, the magazine is moved into the cleaning apparatus before the plasma cleaning process is initiated.
A number of problems have been encountered in such prior art apparatus and methods using plasma for cleaning lead frames. For example, the perforated sides of the lead frame magazines allow contaminants to enter the magazine and deposit on the lead frames or lead frame assemblies during handling. Additionally, during both automated and manual handling of the magazines unwanted materials are deposited on the exterior of the magazine itself. Unwanted material on the exterior of the magazine may cause several potential problems when the magazine is placed in the plasma chamber for cleaning. First, some of the material removed from the magazines may redeposit onto the lead frames. And second, some of the material removed during cleaning operations deposits on the walls and other surfaces of the plasma chamber. Materials deposited on the interior surfaces of the plasma chamber require periodic cleaning. Such materials build up more quickly, and thus require more frequent cleaning, when deposited materials originate from both the exterior surface of the magazine and from the lead frames, rather than just form the lead frames alone.
Prior art plasma cleaning apparatus pass the plasma across the surface of the lead frames in a single direction. Plasma is very short lived. Therefore, the edge of the lead frame nearest the source of the plasma is cleaned significantly better than the edge of the surface furthest from the plasma source. The wider the lead frame, the more significant this problem becomes.
Accordingly, what is needed is an improved apparatus and method for cleaning the surface of lead frames that overcomes the disadvantages of the prior art.